The GABA/A receptor is a hetero-oligomeric integral membrane protein that hyperpolarizes membranes by opening an intrinsic C1-channel that is gated by the neurotransmitter GABA. The receptor contains different classes of allosteric sites which modulate the action of GABA through the interaction of these sites with corresponding effector molecules, such as benzodiazepines, neurosteroids and endogenous peptides. Genes encoding subunits of the GABA/A receptor constitute a complex and tightly regulated multigene family. At least 14 different, structurally related cDNAs have been cloned from adult rat brain, each of which encodes a receptor subunit found in neuronal and glial populations. Although the stoichiometry of subunits needed to form the receptor complex is not known, it is clear that the physiologic and pharmacologic properties of GABA(A) receptors are defined by the combinations and stoichiometry of these subunits and there are reasons to believe that a vast number of receptor subtypes exists in mammalian brain. We have developed a polymerase chain reaction derived assay for quantitating the absolute amounts of each GABA/A receptor subunit mRNA for which a cDNA sequence is available. We will use this assay to analyze the profiles of expression of the receptor subunit mRNAs during development (in vivo) and as cerebellar granule cells differentiate in vitro (Aim 1). This information will be correlated with a comparable in situ hybridization analysis during development in vivo (Aim 2). We will test the hypothesis that cell type specific patterns of expression of the receptor subunit mRNAs are influenced by both heterologous (Aim 3) and homologous (Aim 4) receptor stimulation. In the contest of Aim 5 we will investigate mechanisms through which the trans-synaptic regulation of GABA/A receptor subunit mRNAs may be mediated. The significance of this work derives from the hypothesis that GABA/A receptor subtypes present in discrete neuronal populations specify the level of response that can be imposed on a post-synaptic neuron by afferent synaptic signaling. Moreover, we hypothesize that the expression of these receptor subtypes is physiologically adjusted by normal brain activity and when this tuning fails, function becomes abnormal. An understanding of this functional regulation may offer the possibility for new therapeutic approaches to rectify certain brain function abnormalities.